1. Field of the Invention
The invention relates to non-destructive evaluation (NDE) of inanimate objects, such as solid turbine shafts and the like, by ultrasonic inspection systems and methods. More particularly the present invention relates to industrial ultrasonic inspection systems utilizing phased array inspection probes that are externally oriented about the inspected object (such as a solid turbine shaft) and that utilize Distance-Gain-Size (DGS) reflected waveform analysis techniques to correlate energy of waveforms reflected from discontinuities within the inspected object, with known energies of waveforms generated by a known size flat bottom hole (FBH) and/or side drilled hole (SDH). The discontinuity is thus correlated with pre-acquired reflected energy data (e.g., waveform amplitude) of known size holes.
2. Description of the Prior Art
NDE of an industrial object by an ultrasonic modality identifies discontinuities, such as cracks or voids, by transmission of pulsed sound waves through the object and reception of reflected “echo” waveforms. Often pulse transmission and echo reception are performed by a probe device. The reflected waveform is analyzed for acoustic patterns that are correlated with discontinuities in the inspected object. A discontinuity present in a given material will reflect a different waveform than discontinuity-free homogeneous material. Generally, relative distance between the ultrasonic probe and the discontinuity is a function of elapsed time between probe transmission of the sound wave and reception of the reflected waveform. Discontinuity physical size (i.e., its occupied volume) is indirectly correlated with the echo waveform energy (e.g., amplitude), because reflected energy is impacted by a multitude of physical factors including discontinuity physical size and dimensions, as well as attenuation of the wave energy as it travels through the inspected material.
Reflected or “echo” wave amplitude alone from a single waveform scan orientation may not provide, sufficient information to determine the estimated envelope of physical dimensions and profile of a discontinuity. Dimensional and profile information is useful for making an ultimate inspection determination whether the inspected part is acceptable to use in industrial service. In the past, analysis of a plurality reflected waveforms taken from different respective probe scan positions about the inspected object and variation of transmitted wave frequency/wavelength has enabled inspectors to construct composite spectral and/or visual images of a scanned object that correlate the approximate discontinuity size with that of a known hole size or a plurality of adjoining holes. Depending upon the physical dimensions of the scanned inanimate object and the relative dimensions of discontinuities, ultrasonic images have been constructed of sufficient resolution evaluate potential impact on the inspected part's future use in service, even though the exact physical boundaries of the discontinuity remain unknown.
A traditionally difficult to inspect service part requiring ultrasonic wave transmission over relatively, long distances is a solid steel alloy turbine shaft 10, shown in FIG. 1. Shaft 10 often has a diameter on the order of 500-1500 mm (19.7-59 inches) and often inspected with blades remaining in situ on the shaft. Relatively long sound wave transmission distances through solid steel alloy portions turbine shafts traditionally required relatively high power ultrasonic waves with a relatively high signal-to-noise ratio, all in combination leading to a lower than desired spectral or visual image resolution. It is desirable to inspect at least the inner fifty percent (50%) core internal volume 14 of the solid shaft 10 cross section. In a 500 mm diameter shaft at least a sector-shaped volume U of plus and minus 28° relative to the shaft centerline must be scanned in order to include all of the inner 50% core internal volume 14 of the shaft cross section. Required sector angle needed to scan a shaft inner core volume 14 will vary with shaft diameter.
In known conventional solid turbine shaft scanning methods, as shown in FIGS. 1 and 2, a single “straight beam” probe 20 having a 20 mm (0.79 inch) diameter capable of generating a relatively high energy sound transmission within a narrow cone beam of about 4° is positioned and scans a tangential cross-section of the turbine shaft including the inner fifty percent core volume 14. In order to complete the full tangential scan across the shaft inner volume, the probe is positioned in successive tangential scanning locations A-E. In each individual scan sequence a wedge angle block 22 is interposed between the probe 20 and the shaft surface 16 to orient the scanning beam adjacent to and slightly overlapping the prior scan beam cone. Different angle wedge blocks 22 are utilized at different respective probe scanning tangential positions A-E, necessitating a unique probe and wedge fixture setup for each position. Repetitive setup-scan-prepare the next scan setup sequences are time consuming: often requiring multiple days to complete an inspection on a single shaft. One way to reduce scanning time is to maneuver the probe 20 axially relative to the solid shaft, so that the entire axial volume of inspection interest is scanned before moving the probe to the next successive tangential position for the next set of scans. In this type of known scanning procedure a “matrix” of overlapping scans waveform data (i.e., successive tantential/axial columns of overlapping cone beam emitted by the probe) are combined to construct a single composite scanning plane inspection view of discontinuities in the region of interest. It is preferential to combine scan data from multiple scanning plane views by rotating the shaft to new circumferential positions shown by rotational angle θ and generating a new scanning plane data set for each angular rotational position. Data sets from multiple scanning plane matrices taken about the shaft circumference 16 enable construction of a three-dimensional volumetric image of the approximate correlated size of discontinuities within the shaft, especially focusing on the central 50% internal volume 14.
In industrial ultrasound modalities correlation of discontinuity size with its reflected energy traditionally has been performed by two methods: the reference block method with distance-amplitude-correction (DAC) or distance-gain-sizing (DGS). While the objective of each correlation method is to associate a flaw size echo energy reading with that of a known equivalent hole that reflects a similar energy level, each has relative benefits and disadvantages. DAC requires the use of a test block each time during the inspection. Due to the size of the rotors the DAC has to be performed using a large test block comparable to the rotor diameter. This is not practical for an in service inspection. In contrast, DGS correlation can be performed once in the laboratory and the correlation information is recorded in a spreadsheet with the correlation curves that are needed for the inspection. While the DGS method has been used in industry for a long time, determining the correlation information curves for rotors with large sound path distances (i.e., diameters) require a custom made correlation test block.
In the DGS method a calibration reference block specifically constructed with the same material matching the test object has a series of flat bottom holes (FBH) and normal to the probe scan axis (side drilled holes or SDH). The FBH are drilled at different angles in order to measure the angle amplitude correction (AAC). A 0° beam will have more beam energy than the 30° beam energy for example. All the beams need to be fired with the same amount of energy. The reference block is scanned by the probe. Reflected energy readings for each SDH individual calibration hole in the reference block are measured and converted to energy from FBHs at the same depth as the SDHs and then stored in an analyzer (often in the form of a DGS reference curve of hole size/echo amplitude for a specific distance from the probe). A discontinuity echo energy amplitude is compared manually or automatically in the analyzer with the known reference block amplitude readings for known hole sizes at the closest approximate distance. The closest size reference hole diameter is identified. Alternatively the reference information may be combined in a “pass-fail” curve establishing a maximum reference hole size at any given inspection depth within the tested object. Test objects having discontinuities below the maximum reference, hole size pass the test.
While modern automation systems greatly increase correlation of discontinuity size with a reference hole size, each DAC method inspection requires a first step of taking reference block readings at the actual inspection site before conducting a physical inspection of the test object. Field empirical testing and correlation of discontinuity and reference hole sizes by the DAC method is time consuming and subject to variances in the calibration procedure from one test site to another. An advantage of the DAC method is that all variables impacting correlation are included in the reference block calibration. In the past this advantage of all test variable inclusion has been given great importance when inspecting relatively thick test objects such as solid steel alloy turbine shafts that have required great ultrasonic pulse transmission depths.
In the DGS method, echo amplitude readings are measured for individual holes (FBH and/or SDH) that are arrayed at select distances in a calibration block and permanently stored as a set of reference curves of hole size versus distance in an analyzer. When a field inspection is performed a discontinuity's echo energy reading is compared to one or more hole size curves taken at the same distance from the probe. The discontinuity is correlated with a closest size reference hole's matching curve and/or a designated hole size pass/fail threshold may be predesignated. Discontinuities below a given threshold pass the test for future service use of the tested part. Advantages of the DGS method are elimination of field calibration of a reference block as is required by the DAC method and consistent use of the same test data curves for each inspection sequence. However, a traditional perceived disadvantage of DGS correlation methods for relatively thick test objects, such as solid steel alloy turbine shafts, was whether impact of all factors differentiating probe echo data led to sufficiently accurate correlation of the discontinuity and reference hole size. Inaccurate correlation could result in scrapping of otherwise properly serviceable parts if the test reading overstated the reference hole size or field failure of a component used in service where the correlation understated the reference hole size. Another traditional perceived disadvantage to using DGS correlation methods was the cost and effort needed to construct a large and complicated test block needed to derive the DGS calibration curve data and running the actual correlation tests.
In the past others have suggested use of phased array ultrasonic inspection probes for industrial non-destructive evaluation of components. A phased array ultrasonic probe has multiple transmitter elements that collectively sweep a series of transmitted pulses across a sector-shape swath within a test component. Thus it is suggested that a single phased array probe in a single scanning location can substitute for multiple tangential scans taken with a single element probe. One reported test procedure allegedly utilized a 32 element phased linear array ultrasonic probe and a portable phased array ultrasonic flaw detector, with DAC reference block method for correlating discontinuities and FBH reference hole size.
U.S. Pat. No. 7,017,414 states that a phased array ultrasonic probe may be placed in cavities, such as a hub bore of a turbine wheel, and the transmitted pulses selectively steered to focus on areas of interest within the wheel. The subject patent further states that a DGS technique can relate amplitude of reflected sound from the hub bore of a turbine wheel to amplitude response from known size flat bottom holes (FBH) at varying distances from the probe. It further states that DGS diagram data can be obtained through computer modeling of sound field responses or can be determined empirically using geometrically equivalent calibration blocks containing machined FBH reflectors. However, those skilled in the art know that DGS computer simulation and modeling of DGS calibration curves traditionally was done only for conventional prior art non-phased array probes. It is generally known that radial thickness dimensions from a turbine wheel central bore to its outer periphery and axial thickness are substantially less than the diameter of a solid turbine shaft, and hence generating actual calibration block DGS curves or computer modeled DGS curves for hollow, bored shafts is substantially easier than attempting to do so for large diameter turbine solid shafts.
Thus, a need exists in the art for an industrial NDE ultrasonic inspection system that facilitates quicker and simpler external scanning of interior areas of interest in solid objects without a multitude of repetitive matrix-like scanning passes tangentially across the test object.
Another need exists in the art for an industrial NDE ultrasonic inspection system that facilitates quicker and simpler external scanning of interior areas of interest in solid objects without reference block pre-calibration as is required by DAC techniques.
Yet another need exists in the art for an industrial NDE ultrasonic inspection system that facilitates quicker and simpler external scanning of interior areas of interest without either repetitive matrix-like scanning passes or reference block pre-calibration that quickly generates inspection reference information about relative location and correlated reference reflector size of discontinuities within the central core volume of a solid inspection object, such as a solid steel alloy turbine shaft, with sufficient and accurate resolution to make part serviceability inspection decisions.